Natural gas is ever increasing in popularity as an alternative to gasoline as gasoline prices continue to rise and/or fluctuate. In particular, compressed natural gas, or CNG, is a readily available alternative to gasoline. CNG costs about 50% less than gasoline or diesel, emits up to 90% fewer emissions than gasoline. CNG is made up mostly of methane, and is odorless, colorless and tasteless. It is produced by compressing natural gas to less than 1% of its volume at standard atmospheric pressure, and is drawn from domestically drilled natural gas wells or in conjunction with crude oil production.
Numerous manufacturers offer or are beginning to offer factory-built CNG trucks, step-vans, transit buses and school buses, and more recently, light-duty cars, vans and pickup trucks. An alternative to purchasing one of these vehicles, consumers can convert their existing vehicles to run on CNG. However, the move for consumers to CNG is somewhat hindered, particularly in the U.S. due to the limited number of refueling stations. Expanding the numbers of CNG fueling stations would allow for the increase of CNG vehicles on U.S. roads. However, it will be some time before the supply or infrastructure of public CNG fueling stations meets demand.
Home fueling stations is one solution for currently refueling a CNG vehicle. However, domestic natural gas lines are not compressed. Therefore, a compressor is needed. Companies are beginning to offer CNG refueling stations or compressors that connect to domestic low-pressure natural gas supply, and that utilized domestic power supply. One such refueling station is commercially available as PHILL supplied by BRC FuelMaker. The PHILL refueling station uses a power supply of 220 Volts, with an average electric consumption of 0.85 kw/hr. The inlet pressure of the gas is 17-35 mbar, and the outlet pressure is about 207 bar or 3.000 psig, with a flow of about 1.5 sm3/h. The current cost is about $2500-$5000 for the pump and installation according to publicly available information.
Compressors used to compress natural gas are typically positive displacement reciprocating compressors which utilize pistons driven by a crankshaft. The crankshaft is driven by electric motors or internal combustion engines. The pistons are displaced within a corresponding cylinder to form chambers filled with gas. The pistons reciprocate by moving into and out of the cylinder at a speed determined by the revolutions per minute (RPM) of the crankshaft. As the piston moves into the cylinder, the chamber volume decreases, thereby decreasing the volume of gas, which intern increases the pressure of gas, and the temperature if compressed quickly enough such that heat is not removed at the same or similar rate that it is generated.
Small compressors operate at low-volume and high speeds, and at low horsepower. Low volumes are required because otherwise the amount of forced needed by the piston to compress the gas in the cylinder would be too great to effectively power the compressor. More particularly, because work is equal to force times distance, and horsepower is the rate of work, if a larger volume was utilized, the force and/or distance would increase, thereby increasing the amount of work needed for each cycle, thereby increasing the amount of horsepower to drive the system. Therefore, the speed of the cycle is increased to compensate for the low volume output.
Furthermore, based on the gas laws, compression of a gas increases its temperature, and is referred to as the “heat of compression.” In high speed compression systems, it is assumed that the system is adiabatic, in which the compression is happening so quickly that little to no heat is removed from the system during the compression cycle. In this system, the theoretical temperature rise is calculated to be T2=T1(p2/p1)(k-1)/k where T1 with T1 and T2 in degrees Rankine or kelvins, p2 and p1 being absolute pressures and k=ratio of specific heats. Pressure is related to volume by the relationship p2/p1=(V1/V2)n, where n is typically between 1 and k. Therefore, to keep the heat of compression in control, smaller volumes are preferred.
Problems with prior art compressors are encountered through the high speed, low volume engineering. For example, as described above, the high speed cycling does not allow for rapid heat dissipation of heat generated from the compression of the gas as well as from friction. Multiple, precision, high speed moving parts require heat dissipation and extensive lubrication to extend the useful life of the compressor. Failures in such compressors can include cracking of the piston and/or cylinder, causing a leak in the chamber.
There remains a need for a slow speed, low horse power, high volume compressor that is economically manufactured, and suitable for use as a residential fueling station for vehicles and/or appliances utilizing CNG.